Embodiments of the invention relate generally to electron emissive materials and in particular to electrode materials for electric lamps.
Electron-emissive mixtures containing alkaline earth oxides, specifically barium oxide, have been typically used in mercury discharge lamps. However, the use of barium oxide in metal halide discharge lamps poses certain challenges. The use of barium oxide as a component of lamp electrodes, especially in low-pressure metal halide discharge lamps, is expected to lead to performance issues. This is at least in part due to the reaction of the halide with barium oxide, which can lead to the formation of barium halide. For example, a metal halide discharge material such as indium bromide may react with an emission material such as barium oxide to form barium bromide and indium oxide. It is advantageous to avoid such a deleterious reaction in discharge lamps involving the metal halide emission material, as it may lead to a reduction in the lumen output and life of the lamp.
Typical electron emissive coatings currently used in association with electrodes in many commercial fluorescent lamps contain a mixture of barium, calcium, and strontium oxides (“triple oxide emissive mixture”). Since these oxides are highly sensitive to ambient carbon dioxide and water, they are generally coated on the lamp electrodes initially as a wet mixture suspension of barium, calcium and strontium carbonates containing a binder and a solvent. The wet mixture suspension is then “activated” inside the lamp assembly during the manufacturing process. Activation includes converting the carbonate into an oxide typically by resistively heating the electrodes until the carbonates decompose, releasing carbon dioxide and some carbon monoxide, and leaving behind a triple oxide emissive mixture on the electrode. However, the release of carbon dioxide and carbon monoxide can be disadvantageous as it may lead to changes in the discharge dynamics causing lower luminescence of the lamp. Activation further includes processing the material to a state required for electron emission. Incomplete activation may lead to lamp performance issues like higher ignition voltage, premature cathode breakdown, and loss in light output due to early wall darkening.
Therefore, there is a strong need for electron emissive materials which address one or more of the foregoing problems.